This invention relates to an orbital wastewater treatment system. This invention also relates to an associated method of operating an orbital wastewater treatment system and to a kit for modifying an existing orbital wastewater treatment system.
In a wastewater process employing an activated sludge process, wastewater impurities including domestic wastes, sugars, lipids, proteins, carbohydrates and other nitrogen- and phosphorus-containing materials are decomposable by microorganisms, as is well known in the art. As the impurities are decomposed, a sludge of settled material and microorganisms is wasted from the process either on a continuous or non-continuous basis. The purpose of sludge wasting is to keep solids from building up in the system. Sludge from the process is normally transported by pumping to a digester for treatment prior to landfill or other disposal so as to reduce the volatile organic content of the sludge, reduce the sludge volume, reduce the pathogenic organisms present in the sludge, reduce its odor potential and improve sludge dewaterability, and for other reasons of lesser importance. Various prior art types of digesters and various digestion and stabilization processes have been proposed and used.
An early circa 1960 installation by Pasveer for wastewater, i.e. primarily sewage, purification by the activated biological sludge method included a closed circuit or ditch with a horizontally-rotated brush rotor used for adding needed oxygen (air) to the sewage and moving the sewage in circulation. In U.S. Pat. No. 3,510,110, an orbital system employing an elongated tank with central partition was disclosed which employed a vertically-rotated surface aerator located at the end(s) of the partition wall for both aerating the sewage and circulating the sewage around the channels formed by the partition wall and the sides of the tank. This latter system has had great commercial success with over 1000 plants in operation world-wide (ranging from less than 1 MGD capacity to one of over 10 million population equivalent) with over 600 plants in operation or in various construction phases since 1976 in the United States up to the present time. Sold under the trademark “Carrousel®”, the high popularity of the system is due primarily to its cost-effectiveness, simplicity of design, ease of operation and maintenance, and excellent effluent quality. It can treat raw domestic water to EPA advanced secondary standards without primary clarifiers or effluent filters. With extended aeration, it produces a highly stable water sludge requiring little or no further processing prior to disposal. Carrousel® orbital wastewater treatment systems can be designed to have a power turn-down of 50 to 85 or 90%. Aerator drive horsepower can be varied from 100% of installed capacity to as little as 10% without loss of mixing and continuing sufficient mixed liquor channel velocity. This power turn-down flexibility provides an ability to closely match oxygen input to the mixed liquor to oxygen demand of the microbes acting to degrade the sewage, without loss of mixing and movement. In one of the largest U.S. installations, over 25 MGD of sewage is treated in four units having twenty aerators utilized to aerate and circulate sewage through twenty-four channels formed by twenty partitions and exterior encircling concrete walls forming four tanks.
Improvements in Carrousel® orbital wastewater treatment systems are disclosed in U.S. Pat. Nos. 4,869,818, 4,940,545 and 7,186,332. In general, each Carrousel® orbital wastewater treatment system sold under the trademark denitIR® includes a tank having at least one partition that defines an anoxic zone and an aerobic zone that are operated in accordance with the modified Ludzack-Ettinger (MLE) Process. The partition also defines passages from said aerobic zone to said anoxic zone and from said anoxic zone to said aerobic zone. At least one impeller/aerator is located in said tank for moving mixed liquor under process about said tank and for increasing the dissolved oxygen content of the liquor in the aerobic zone. A manually adjustable flow-diversion gate is provided at the passage for controlling the recycling of nitrates to the anoxic zone from the aerobic zone. The aerator is efficient in oxygen transfer and mixing so as to maintain solids in suspension while varying oxygen input so that the main channel flow reaches an anoxic condition as it passes the flow-diversion gate. In the anoxic basin or zone, screened and degritted influent and recycled activated sludge are mixed with nitrified mixed liquor, providing optimized conditions for high rate denitrification, pursuant to the MLE Process. Bacteria feed on the carbon-rich influent, using molecular oxygen from the abundant nitrate to drive metabolic reactions. Nitrate is first reduced to nitrite, then to nitrogen gas, which is subsequently stripped in the aeration basin. In the process, portions of the alkalinity and oxygen consumed during nitrification are restored.